1. Field of the Invention
The present invention relates to a wireless transmission device for transmitting signals in parallel, using a plurality of transmission units, and a wireless receiving device for receiving the signals transmitted.
2. Description of the Related Art
A scheme for transmitting a plurality of signals in parallel using a plurality of transmission units each having a transmitter and antenna is known as a technique for increasing the rate of wireless communication from, for example, A. Paulraj, R. Nabar and D. Gore, Introduction to Space-Time Wireless Communication, Cambridge University Press, Cambridge, UK, 2003. In this scheme, to-be-transmitted information is divided into a plurality of information units by serial-to-parallel conversion, each information unit is subjected to, for example, encoding and modulation, thereby generating a plurality of transmission signals and distributing the transmission signals to the respective transmission units. Each transmission unit performs digital-to-analog (D/A) conversion and/or frequency conversion on the received transmission signal, thereby generating a radio frequency (RF) signal and transmitting it through the corresponding transmission antenna after amplifying it.
The signals transmitted in parallel from the transmission units are subjected to multiplexing on propagation paths in space, therefore, the receiving side must take measures against mutual interferences. In order to avoid the interferences, a plurality of receiving units that have respective antennas and receivers may be used to separate the transmitted signals. Even a single receiving unit can simultaneously detect a plurality of signals without mutual interferences by estimating, from the received signals, the signals whose joint probability density function is maximum.
In the above-described wireless communication system, the transmission rate can be enhanced in accordance with the number of transmission units, without extending the frequency bandwidth for communication. Accordingly, compared to the case of using a single transmission unit, the frequency efficiency is increased, and the throughput is enhanced.
Since the antennas included in the respective transmission units are located at different positions, the signals received at the receiving side, which correspond to the respective transmission signals, have different power levels. Accordingly, the each channel has different capacity. In light of this, to enhance the efficiency of transmission, the signal transmission rate, more specifically, the modulation and coding scheme (MCS), is optimized.
In particular, in the so-called multi-input multi-output (MIMO) system, in which signals are transmitted, utilizing spatial multiplexing, from a wireless transmission device with a plurality of transmission units, and received by a wireless receiving device with a plurality of receiving units, a number of propagation paths, i.e., channels, which have different channel responses, are formed between the transmission and receiving devices. Therefore, if the transmission rate is changed appropriately, highly efficient transmission can be realized.
In this case, however, it is necessary to notice, to the receiving side, the transmission rate of a signal transmitted from each transmission unit, i.e., the MCS of each transmitted signal. Specifically, it is necessary to include, in the header of each transmission signal, information indicating the MCS of signals transmitted from each transmission unit. If a large number of MCS sets are used, the data length of the header is inevitably increased, thereby reducing the throughput.